Insemination of mammlian spermatozoa aged in the male or female tract is followed commonly by pre- and post-implantation embryopathies. These are believed generally to stem from age-induced defects in the sperm nucleus. We challenge this concept because of what is now known about the stable nature of the eutherian sperm nucleus and because the evidence for it does not distinguish a likely alternative that this also favors delayed fertilization - a known root of such anomalies. The research proposed will assess critically the true vulnerability of the mammalian sperm nucleus to aging in vivo. First, the developmental potential of early pronucleate rabbit ova fertilized soon after ovulation by spermatozoa subjected to prolonged aging in the male tract, will be compared with that of parentally-matched control zygotes in the same female. Using this same assay, the special possibility the prolonged survival of rabbit spermatozoa in the rat uterus provides to extend their fertile life to double that seen in the homologous uterus, will be exploited to exaggerate any potential for age-related genetic defects after ejaculation. Cytological and karyological analysis of embryos will be performed where indicated. Whether or not the thiolrich nature of eutherian chromatin may allow further oxidative cross-linking and so excessive stabilization during aging in vivo will be evaluated against controls by: 1) comparison of the respective rates of chemically induced decondensation; 2) measurement of the specific affinity of the nucleus for iodo-acetamide, and -SH affinity reagent; 3) assessment of the synchrony of male and female pronucleus development following fertilization by aged spermatozoa. Finally, the possibility that other age-related changes can occur in sperm chromatin will be assessed in collaborative pilot studies using flow cytometry. The research proposed has specific importance for evaluation of the risk to the human embryo created by the common asynchrony between insemination and the time of ovulation. It will also allow us to understand more precisely the mechanisms through which the putative paternal contribution to embryonic pathology can be expressed.